Preemptive Control of Multiply Actuated Processes: Application to Moisture Content Control in Paper Manufacturing Using Surrogate Measurements

Abstract

Many manufacturing processes involve the successive processing of the substrate at multiple station on a transport medium, with the hope that at the end of the process, the product has the desired property. Paper manufacturing is an example in which over 90% of the water from pulp is sequentially removed through gravity, vacuum dewatering, pressing, and thermal drying. The consistency and uniformity of the moisture content at the end of process is important for paper quality. Current strategy for the control of moisture content uses a feedback sensor at the end of the process to adjust the dryers. This introduces a long deadtime and causes excessive use of the dryers, which translate to limitations in performance, robustness and inefficient energy usage. In this paper, we investigate a new control approach in which in-process moisture contents are estimated using air-flow as surrogate measurements, and the pressure settings in the multiple vacuum dewatering boxes are adjusted according to the surrogate measurements. A preemptive control algorithm is developed which has the ability to decouple and eliminate the effects of the disturbances that occur upstream in the process from downstream. Robustness analysis and simulation studies suggest that as long as the surrogate measurements are accurate, the proposed control scheme will be robust and accurate.